It is known to provide in such apparatus one or more inductive sensors which generate electromagnetic fields in a test region through which a coin is arranged to travel. The coin influences the field to an extent dependent upon the dimensions and/or material of the coin. The inductive sensor, and the circuit to which it is coupled, may be arranged so that the influence of the coin on the electromagnetic field is predominantly determined by the coin material, the coin diameter or the coin thickness. The inductive sensors tend to be of a size comparable to that of the coins which they are intended to validate, to ensure sufficient sensitivity. This, coupled with the fact that the electromagnetic fields generate eddy currents throughout the body of the coin, results in the inductive sensors tending to be responsive to the bulk or average properties of the coin. Some coins, however, are formed of a composite of two or more materials, such as a central core of a first metal surrounded by one or more outer rings of a second or respective further types of metal. Conventional sensors cannot easily discriminate between these bimetallic (or, in general, multi-metallic) coins and homogeneous coins made of a material which influences the sensor to substantially the same extent as the average influence produced by materials of the non-homogeneous coins. Also, because the sensors can detect effects on the electromagnetic field over a large distance, they tend to be less sensitive to the precise position of the coin and therefore not particularly accurate at measuring coin geometry.
WO91/15003 discloses a validator for bimetallic coins in which first and second relatively small Hall effect sensors are provided, at different heights from, and positions along, a coin track so as to sense different portions of a coin simultaneously and the sensor outputs are thresholded to validate a coin.
U.S. Pat. No. 4,742,903 discloses a validator for bimetallic coins in which the outputs of several sensors along a coin track are separately derived and supplied, in time-division-multiplex form, for separate processing.
U.S. Pat. No. 4,870,360 discloses a coin validator in which a first Hall effect sensor is positioned on a coin track, and a second is positioned away from the track or adjacent to a reference coin, and the difference between the two sensor outputs (set to be null in the absence of a coin) is used to validate multi-metallic coins.
FR2538934 discloses a coin validator for testing for a single coin type in which first and second sensors are positioned along, at different heights from, and on opposite sides of, a coin track so as to sense different portions of a coin, and the sensor outputs are adjusted so that the difference there between is zero in the presence of a valid reference coin.
A test coin is validated by detecting the exact moment when a coin is symmetrically positioned adjacent to both sensors, and then sampling the magnitude of the difference between the sensor outputs and rejecting a coin if the magnitude is significant.
It does not validate bimetallic coins. Even if a multi-metallic coin were to be placed into the coin track, the sampled difference reading used to validate a coin represents the sensor outputs only at the instant when the coin is symmetrically positioned relative to the sensors, so that the coin material detected by each sensor would be identical, and the arrangement would therefore not be sensitive to the material differences within the multi-metallic coin.